Various inspection methods are available to detect indications in materials. These indications are anomalies in materials and may be classified as defects if the anomalies are serious enough to be categorized as a problem during the life of the component of which the material is comprised. When the indications are not serious, they may be classified as acceptable anomalies as their size and shape have been evaluated and deemed not to affect the operation of the component over its useful life.
There are various inspection techniques available. Some of these include visual inspection, eddy current inspection, liquid penetrant inspection, magnetic particle inspection, ultrasonic inspection and radiographic inspection. Each of these techniques has its benefits and limitations and frequently more than one of these inspection techniques are used together to detect and evaluate anomalies in a component.
The oldest of these inspection techniques is visual inspection. Clearly, visual inspection is utilized to evaluate the exposed surface of a component and will disclose indications that are visible from the exterior of the object. The limitations of visual inspection are that it cannot disclose indications that are below the surface of the exterior of the object under inspection and hidden from sight. Visual inspection also may not be able to resolve very small indications that may be open to the exterior surface of the object.
Liquid penetrant inspection utilizes a highly visible fluid liquid applied to the surface of the object. This highly visible liquid is drawn into any surface opening by capillary action. The surface is wiped clean and a powder material having a color that contrasts with that of the liquid is applied to the surface of the object, the powder drawing the highly visible liquid from the openings into which it has penetrated, again by capillary action, onto the surface. The amount of liquid penetrant drawn back to the surface is an indicator of the size or volume of the indication. The limitations of the liquid penetrant inspection are that the indications must be open to the surface and the indications must be fairly small with tight openings to the surface, as a shallow, large indication will have penetrant removed when the surface is wiped clean. Of course, shallow, large indications are usually detected by the visual inspection, which complements liquid penetrant inspection.
Eddy current inspection applies an electric current across the surface of an object. The advantage of the eddy current inspection is that it can detect subsurface indications that are not open to the surface. The limitation of the eddy current technique is that it can only detect indications near the surface of the object under inspection (referred to as near surface indications), as the eddy current is a surface current that does not otherwise penetrate into the object.
Magnetic particle inspection utilizes fine magnetic particles suspended in a highly visible carrier solution. Magnetic particle inspection is limited to ferrous articles. The solution holding the suspended particles is applied to the surface of the article to be inspected and the carrier solution and the magnetic particles are drawn into any opening in the surface by capillary action, similar to liquid penetrant inspection. Excess solution is removed from the surface of the article and an electric current is applied to the surface of the article usually in at least two directions, preferably perpendicular to one another. The electric current results in a magnetic field being formed across any surface openings (and in some cases near surface openings), which draws some particles and visible solutions to the surface. Usually, the solution is a fluorescent solution and the inspection utilizes an ultraviolet or black light to irradiate the surface, making it significantly easier to see the fluorescent solution. The limitations of the magnetic particle penetrant inspection are that the article that is to be inspected must be magnetizable, typically ferrous, indications must be open to the surface and the indications must be fairly small, as a shallow, large indication will have penetrant removed when excess penetrant is removed from the surface. Of course, shallow, large indications are usually detected by the visual inspection. In addition, when contact electrodes are used by the technician to apply the current to the surface of the article, it is possible that arc strikes may occur to the article surface if care is not exercised by the technician. These arc strikes may in certain applications be deemed as damage.
Radiographic inspection utilizes x-rays for detecting anomalous indications in articles based on density differences between the indications and the base materials. This inspection technique is generally very useful when the indications are not otherwise detectable and the density difference between the base material and the indication is great. For example, radiographic inspection is useful to detect cracks or porosity in an article or a tungsten inclusion in a weld. This inspection technique utilizes high energy, short wavelength electromagnetic radiation that passes through different materials at different rates. More of the high energy waves are absorbed dense materials, so materials such as tungsten do not allow the same quantity of waves through as for example, porosity or cracks, which transmit all of the waves. A detector measures the waves transmitted and can pinpoint the density differences. Limitations are that radiographic inspection is not very useful for detecting imperfections in multi-phase materials such as precipitation hardened materials. The precipitates usually are of a different density than the base material, and the relatively uniform distribution of particles make it very difficult to discern an indication in the base material and distinguish it from precipitates. In addition, flat or plate-like indications, when oriented perpendicular to the direction of the incident x-rays also can be difficult to detect, particularly when the thickness of the indication when compared to the overall thickness of the material being inspected is small. However, when the material can be inspected from multiple directions, such indications are readily detectable since a plate-like indication, while having little thickness when approached from one direction, usually has ample thickness when approached from a second direction at a high angle, preferably in the range of 45-90° from the first direction. The detected indications can be compared to established standards to determine their acceptability or their need to be removed.
Ultrasonic inspection utilizes ultrasonic waves which are transmitted through a metallic article to detect indications. Ultrasonic inspection utilizes timed pulses of wavelengths generated in the frequency range of 20 KHz to 20 MHz and transmitted through the bulk of the material under inspection. When the pulse echo technique is utilized, a wave is transmitted from a transducer, reflected from a back surface of the article and returned to the transducer. The amount of time required for the wave to travel through the article is known. A wave returning to the transducer in a shorter period of time is indicative of an indication. Usually, the amount of energy reflected back to the transducer provides the operator with an idea of the size of the indication. When the article can be inspected from a plurality of directions, it may be possible to map the size, shape and location of detected indications. The determination as to the acceptability of an indication may be made on the amount of reflected energy from the indication as well as the size, shape and location of the indications. Ultrasonic inspection is utilized with surface inspection techniques such as liquid penetrant or magnetic particle inspection since ultrasonic inspection is limited in its ability to resolve surface indications due to the large front pulse reflection and back surface reflections. These reflections are usually so large that they mask any reflections from indications that may be present at or near these surfaces. In addition, the frequency of the ultrasonic waves limits the size of the indication that can be resolved. One of the limitations of ultrasonic inspection is that it may not be able to resolve small indications in the path of the applied wave simply because a small indication may not reflect sufficient energy, or the amount of reflective energy may not be a true representation of the size of the indication. Another limitation of ultrasonic inspection is that it cannot be utilized to inspect precipitation hardened materials, simply because precipitates reflect ultrasonic energy making it impossible to discern any anomalous indications and distinguish them from the precipitates. If inspection is to be performed on such materials, it must be accomplished before the precipitates are developed. Similarly, materials including multiple phases, such as particles suspended in a metallic matrix also cannot be inspected due to the reflected energy. The ultrasonic techniques have also required a liquid couplant between the transducer/transceiver and the article, because air is a poor transmitter of ultrasonic energy. However, recent advances in transducer technology have solved this problem, at least for certain wavelengths.
In modern military aircraft, components are fabricated from ceramic matrix composite (CMC) materials in order to lighten the weight of the aircraft. These CMC materials form CMC structures that may replace heavier metallic materials in the same application. While inspection techniques, such as the ones described above are available for use to test the adequacy of metal structures, aside from visual inspection, the same techniques are not available to detect anomalies in CMC structures used in aircraft structures. What is needed is an inspection technique that can detect anomalies in ceramic matrix composite structures, such as lack of bonding, voids, and impedance differences, which may indicate a structural imperfection, so that the structural imperfection may be further evaluated to determine its acceptability or the need for repair.